Modern archaeomonads indicate sea-ice environments

Diatoms, radiolarians, silicoflagellates, ebridians and archaeomonads are the primary groups of pelagic organisms in siliceous marine deposits, with only the archaemonads regarded as being absent from modern oceans and sediments. They remain a poorly known group of siliceous nanofossils in Cretaceous to Upper Tertiary marine sediments. We present here the first report, from any modern oceanic environment, of pelagic cysts that conform to the descriptions for Archaeomonadaceae. These cysts occurred in seawater, sea ice and marine-surface sediments, demonstrating the group is still extant. Furthermore, their distribution in modern sediment suggests they may be useful guide fossils for sea-ice environments as they occur abundantly only in polar or near-polar water where sea ice is present. Few other microorganisms seem to have such a close and exclusive association with sea ice, thus archaeomonads deserve increased attention as indicators of ice conditions for deep-water marine environments.